1. Field of the Invention
The present invention relates to a flexible meshing type gear device and in particular to a flexible meshing type gear device which has a rigid internal gear of high wear resistance.
2. Prior Art Description
A typical flexible meshing type gear device comprises a circular rigid internal gear, a flexible external gear arranged inside the circular rigid internal gear, and a wave generator for deforming the flexible external gear into, for example, an ellipsoid to mesh it with the rigid internal gear partially. The wave generator is driven by a motor to move the meshing portions between the gears circumferentially, whereby relative rotation therebetween is occurred in accordance with the difference in tooth number between the gears.
The circular rigid internal gear is normally fixed to a housing of the device wherein relative rotation is output from the flexible external gear. Typically, the difference in tooth number between the gears is 2n (n: positive integer), and, for example, the tooth number of the flexible external gear is two fewer than that of the circular rigid internal gear. In this case, the reduction ratio of the flexible meshing type gear device is expressed by the following equation. EQU i=1/R=(Zf-Zc)/Zf=-2/Zf
where Zc: tooth number of the circular rigid internal gear, and
Zf: tooth number of the flexible external gear.
Provided that Zc=202 and Zf=200, the reduction ration i becomes -1/100 in which symbol `-` means that the rotational direction of the output is opposite against that of the rotational input, while symbol `+` means that the output rotational direction is the same as the input rotational one.
In a flexible meshing type gear device, the respective external teeth of the flexible external gear are repeatedly deformed in the radial direction to mesh with the internal teeth of the rigid internal gear intermittently. Thus, the teeth of both gears suffer from much more intensive wearing than those of general gear transmission mechanisms. In order to enhance wear resistance of a flexible meshing type gear device, it is necessary to provide the internal and external gears with a high wear resistance. However, a satisfactory surface treatment which can be applied to the internal and external gears has not been proposed to enhance the wear resistance of the internal and external gears without causing defects such as deterioration of dimensional accuracy of gears, the lowering of fatigue strength of the flexible external gear and the like.